New approaches to enhance the delivery of biologically active macromolecules across the blood-brain barrier (BBB) are being investigated. After working with hyperosmolar blood-brain barrier disruption (BBBD) with mannitol that has been previously identified to increase delivery of relatively large biological agents to normal brain and to tumors, we investigated the possibility that nitric oxide (NO) donors might be used to selectively open the blood-brain barrier in tumors, but not in normal brain. We found that a short-acting NO donor selectively increases the capillary permeability to 14C-aminoisobutyric acid (14C-AIB) in rat C6 and 9L tumors without affecting the normal brain and without having systemic affect. The NO donor is being evaluated for toxicity and for its ability to increase the permeability of the blood-tumor barrier to molecules of a wide range of molecular weights at different doses and infusion regimens. The results indicate that this approach might be an effective new way of opening the BBB in patients with brain tumors to enhance delivery of biologically active macromolecules, including chemotherapeutic agents, genetic vectors, etc., to malignant brain tumors for therapy. Since NO is an endogenous biological agent, we do not expect toxicity associated with its use to open the BBB.